Delusional disorder, once termed paranoia, was an important diagnosis in the late 19th and early 20th centuries, and only in 1987 was it reintroduced into modern psychiatric diagnosis after being subsumed with schizophrenia. This book provides a comprehensive review of delusional disorder for psychiatrists and other clinicians. Beginning with the emergence of the concept of delusional disorder, the book goes on to detail its manifold presentations, differential diagnosis, and treatment. Many instructive case histories are provided, illustrating manifestations of the various subtypes of delusional disorder, and related conditions in the paranoid spectrum. This is the most wide-ranging and authoritative text on the subject to have appeared for many years, and the first to suggest, based on the author’s extensive experience, that the category of delusional disorder should contain not one but several conditions. It also emphasises that, contrary to traditional belief, delusional disorder is a treatable illness.